Field
The present invention relates to a power amplifier capable of suppressing thermal runaway even when a plurality of FET cells are connected in parallel.
Background
Power amplifiers used for radio communication or the like are required to operate stably over a wide range of ambient temperatures. Furthermore, operating power exceeding 40 W is required for mobile communication base stations, and since large power is consumed at power amplifiers, it is particularly important that the power amplifiers should operate stably even at high temperatures.
With FETs often used for power amplifiers, the source is grounded, a negative voltage is applied to the gate terminal and a positive voltage is applied to the drain terminal. A drain current is controlled by a gate voltage, and as the gate voltage increases, the drain current also increases. With actual devices, a leakage current flows backward from the gate terminal, often causing thermal runaway at high temperatures.
A mechanism of thermal runaway is as follows. Since the gate leakage current increases as the operating temperature increases, the gate voltage at the gate terminal increases at high temperatures and the drain current increases. As a result, power consumption at the FET increases, causing the power amplifier to fall into a state in which positive feedback is established with the amount of heat generated increasing and the operating temperature increasing. For this reason, there is a demand for preventing thermal runaway during high temperature operation and it is preferable that a compensation circuit be provided in the power amplifier for downsizing of a transceiver.
In response to such requirements, there is a proposal to use a resistor with a large temperature coefficient for a resistor connected between a gate terminal and a grounding terminal in a bias circuit that supplies a gate bias obtained by dividing a supply voltage by two resistors in a conventional power amplifier (e.g., see JP 2000-349563 A). Since a resistance value of the resistor increases at a high temperature, a voltage applied to the gate terminal decreases. Therefore, it is possible to suppress an increase in a drain current due to an increase in a gate leakage current at a high temperature and thereby suppress thermal runaway.